To overcome the difficulty of field fault diagnosis in the complicated circumstance of construction machinery, the distributed expert system of fault diagnosis is developed in this paper. According to the technical requirements, the hardware platform based on smart industrial portable data terminal is performed. Meanwhile, the modular software of the fault diagnosis expert system is implemented. Its hardware is mainly composed of intelligent portable field testing unit as well as the central control computer. The software platform mainly consists of PDA modules for field fault testing and diagnosis, expert system module, maintenance support and data query module, training module, help module and report printing module. The work has greatly enhanced the capability of field fault detection, diagnosis and maintenance support in military units.